Architectural lighting designers, faced with the task of uniformly and efficiently illuminating the playing surface of a sports field while minimizing the amount of light spilled into the seating portion of the sports field, have utilized individual lighting fixtures that would be of a type that utilized a large light source having an arc tube disposed within an outer envelope that was oriented along the central axis of the light source. The light output pattern of such a fixture would be of a circular shape such that, once directed onto the playing surface of a sports field, resulted in an essentially elliptically shaped light pattern with varying amounts of light intensity around such elliptical shape. This approach suffers in terms of light utilization and the amount of spill light experienced; that is, the amount of light that goes beyond the playing surface and into the eyes of the spectators and in worst cases, beyond the confines of the sports field to surrounding homes and businesses. Because this fixture utilized a light source along the central axis of the reflector with little regard for the amount of light exiting the fixtures at high angles thereby resulting in such spill light, the size constraint of an outer jacket to the light source was not of critical consideration and such outer jacket could be utilized for the purpose of UV filtering from the arc tube disposed within the outer jacket. Although this approach has proven effective in providing a means of filtering UV radiation emitted by the high intensity light source, the consequences of such an approach in terms of light utilization and spill light has had adverse effects. For instance, since the elliptical light pattern results in an inefficient combination of patterns at the playing field surface, there is the need to increase the actual number of fixtures needed to illuminate a particular area and the need to provide external light directing devices such as glare shields and louvers. U.S. Pat. No. 4,725,934 issued to Gordon et al on Feb. 16, 1988, discloses a fixture using an outer jacketed light source disposed along the longitudinal axis of a reflector as well as external louvers and a glare shield for redirecting light otherwise falling at higher output angles than desired. A sports lighting luminaire presently offered by GE Lighting Systems Department of General Electric Company under the product name "UltraSport" provides a solution to the problem of efficiency of light output and the reduction of spill light or glare. In attempting to improve the light delivery characteristics of sports lighting luminaires, the assignee of the present invention has developed the new sports lighting luminaire wherein a high intensity light source is utilized within a parabolically shaped reflector. In order to prevent the emission of UV radiation from such light source, the sports lighting luminaire uses a tempered glass cover for filtering UV.
One problem with the use of a separate cover member to serve the purpose of UV filtering is that, should the cover be broken for instance by vandalism or by accident relating to the sporting event at the field, and the light source remain intact and operating, UV would be emitted from the luminaire. It would therefore be advantageous that, in the event of breakage of the cover member to the luminaire, a means could be provided to insure that the light source is prevented from operating and emitting UV. One approach to solving this problem can be found in publications by Philips Lighting relating to their sports lighting luminaire identified as the "ArenaVision" luminaire. In this approach, a wire mesh grid is disposed within the glass material of the cover member similar to the type of safety glass previously utilized in commercial retail operations in doors and windows. Though effective for insuring the integrity of the glass cover and preventing the glass cover from scattering away from the open end of the luminaire, this approach has the adverse effect of blocking or obscuring the light output from the luminaire even when the cover is intact Furthermore, since this technique is directed to maintaining the positioning of the glass cover even in the event of breakage, rather than interrupting the operation of the light output, it could be possible that portions of the glass cover could be displaced resulting in some leakage of UV. It would therefore be advantageous if an approach could be used that would continuously monitor the integrity of the glass cover but at the same time, not obscure light output therethrough when the glass cover was intact under normal conditions.
It can also be appreciated that if one took the approach of monitoring the condition of the glass cover so as to provide such condition as a logic input to the ballast arrangement used to power the light source, one could positively insure that the shutdown of the lamp operation under a broken glass cover condition, did occur. Therefore, not only would it be desirable to prevent the displacement of broken glass pieces by some means other than a light blocking wire mesh grid technique, it would also be beneficial to positively utilize the information that the glass has been broken as an operating input to the ballast arrangement of the luminaire. In providing a circuit control based on the condition of the cover glass, the cost of such an addition of logic elements, the number of components needed to perform this logic check, as well as the reliability of such a circuit would be of considerable importance. Therefore, it would be advantageous if a circuit to verify the integrity of the cover glass and thereby serve to insure the shutdown of the light source in the event of a broken glass condition, could be provided in an manner that was cost effective, used a minimum number of components and was extremely reliable in operation.